In order to seal bell-and-spigot type pipe connections on concrete-pipes, a so-called roll-in joint is still generally used in practice. The sealing ring is mounted on the spigot and pushed into the bell together with spigot, causing the ring to roll in until it reaches roughly the center of pipe connection. Uneveness along the rolling paths might impede uniform rolling of the sealing ring causing local distortion and reduction in diameter, which in turn might give rise to leakages. Furthermore, thinned sections of sealing rings have become lengthened and might protrude into the gap at the front face of the spigot with the consequence of total loss of sealing effect.
With sliding joints the seal is backed-off by a shoulder on the spigot as shown in FIG. 2 of European patent application No. 16,275, and a lubricant is used for easing assembly of the pipes and preventing abrasion. A disadvantage arises in certain applications causing the lip to be relieved when the pressure inside the pipeline builds up.
There is also the possibility of fixing the sliding seal in the bell, as shown in German patent application No. 29 35 392, whereby the seal is applied as a permanent insert when the pipe is cast. During production it can happen that water seggregating from the fresh concrete accumulates between the seal and its seat, leaving behind an airpocket after evaporating. After such pipes have been laid for the purpose of conveying fluids, the latter can escape through the pockets, seeping out round the back of the seal. A further disadvantage is the fact that the sealing element material ages if stored for a long time and the avoidable storage of seals ties up capilal unnecessarily. Elastomer, though a very effective material for seals, is deleteriously influenced by oxidants, particularly by ozone. Another drawback arises when concrete pipes are being manufactured with constantly deviating dimensions. In that case the integrated seal is either over- or undersized with regard to the seal gap size.
With yet another sliding seal installed in the socket, as shown in FIG. 5 of European patent No. 16,275, the sealing element is provided with a retainer having an extended diameter which is seated in a recess or groove inside the socket. The sealing lip is situated radially opposite to the retainer and faces the fluid to be sealed. Growing internal pressure also increases the radial pressure on the sealing lip. The production of concrete bell-pipes comprising a recess or groove is not described; since reference is made to clay pipes, it is assumed that the groove is pressed into the workable clay prior to burning. This method cannot be adapted to concrete pipes.
Deviations from nominal dimensions frequently occur with concrete pipes. Each batch is either too thick or too thin so that a specific seal size is not sufficient to cover a nominal size range on concrete pipes.
An object of the invention is to find a production method for concrete pipes with which the seal may be mounted in the bell as soon as the concrete-mixture is cured or relatively late, if necessary, immediately prior to pushing the pipe ends together, depending upon the manufacturer's choice.
A further object of the invention is to make it possible to select appropriately sized seals to suit individual batches.
Still a further object is to allow the concrete pipes to be suitable for adapting different types of seals, i.e. sliding joints, roll-in joints.
Concrete pipes are normally cast with the bell at the bottom, whereby "cast" implies any kind of placement and compaction method used in order to charge the mould with concrete. The mould usually comprises a tubular outer mould and a core consisting of a tubular inner wall and a so-called base-ring, herein termed the bell-forming core.